This invention relates to AM, FM, TV, Radar etc., transmitters and the means for reducing the energy level of the undesired rf harmonics at the output of the transmitters. The invention is especially useful for high power transmission systems where the size and cost of conventional means for reducing the magnitude of the undesired rf harmonics can be substantial.
Radio, Radar, Television and other wireless communication and navigation systems must restrict their bandwidth so as to avoid interfering with other users of the radio spectrum. One mechanism for causing interference is by radiating rf harmonics i.e., bands of rf power centered at exact multiples of the desired radiated signal's frequency. For example, if a AM broadcast station is assigned a carrier frequency of 660 kHz its 2nd harmonic will fall at 1320 kHz and its 3rd harmonic at 1980 kHz, etc. These harmonics can, if they are of sufficient magnitude, interfere with stations which operate at, or close to, the frequency of the harmonics.
Accordingly, regulatory agencies require stations to severely limit the radiated power of such harmonics. For example, the U.S. Federal Communications Commission requires all harmonics to be 43+10 log P decibels, but not more than 80 db, below P, the unmodulated carrier power of the station in watts.
In order to achieve relatively high efficiency, high powered rf amplifiers use tubes or transistors biased to conduct current less than the full rf cycle. This non-linear operation produces significant levels of harmonics that must be greatly attenuated prior to reaching the associated antenna. The present invention provides means and methods for providing improved methods and means for providing attenuation of rf harmonics.
The conventional prior art method of attenuating harmonics is to use a bandpass filter comprising tuned circuits which are tuned to the desired frequency, generally, but not always, the fundamental frequency component. (Sometimes the final amplifier is used as a frequency multiplier, in which case the tuned circuit is tuned to the desired harmonic and the tuned circuit is used to attenuate the fundamental and the remaining undesired harmonics.) This arrangement is effective, however the amount of attenuation per tuned circuit section is limited because of a number of reasons including;
(1) The higher the selectivity factor, Q, the higher the circulating current which increases the size and cost of the inductance and capacitor in the tuned circuit. PA0 (2) The higher the selectivity factor, the greater the attenuation of the higher frequency sidebands of the modulated wave. PA0 (3) The higher the selectivity factor, the lower the efficiency. Thus for a single tuned circuit filter the efficiency of the tuned circuit in passing the rf wave is: ##EQU1## where Q unloaded is the Q of the tuned circuit without the output load connected and Q loaded is the Q with the load connected. Typical values of Q unloaded are 200 to 800 and Q loaded 3 to 15. If the Q loaded is 13 a typical class C amplifier will have a 2nd harmonic down only approximately 30 db. For an Q unloaded of 260, five % of the total power is wasted.
Push-pull amplifiers are useful in reducing the level of even order harmonics, but such circuits have a tendency to be unstable and do not reduce odd order harmonics.
Other circuits, such as Pi networks, are more effective than simple tuned circuits, but the difference is not substantial enough to solve the problem.
While there are and have been a number of products in the marketplace that manually and or automatically null harmonics (for example distortion meters) no r.f. harmonics attenuator following the method and means as disclosed herein is known. It is noteworthy that the problem this invention treats was known since the very earliest days of radio engineering. Because the subject invention can save approximately 30% of manufacturing costs of high powered radio and TV transmitters, can substantially reduce the size of the overall transmitter, and can significantly reduce the amount of electric power consumed, the invention solves a long standing need and has substantial utility.